Transportation officials in New York suspended overnight track maintenance on the city's transit system this weekend as workers combed the underground network. All 468 New York City subway stations have been searched, and aviation, harbor and canine officers are mobilized and deployed periodically, the New York Police Department said.

The teenager, who is unable to communicate verbally, is fascinated by trains, his family said, and officials have shifted their search underground, a spokesman for the New York Police Department told CNN on Sunday.

Searches of train stations, tracks and tunnels were also conducted late last week, Paul J. Fleuranges of the Metropolitan Transit Authority said.

Though there are no leads, Daniel Oquendo Jr. fears his brother may have been taken. He told CNN affiliate WABC, "It hurts. It's very stressful to know that someone would take an innocent, poor, mentally disabled child and take advantage of them."

Police Commissioner Ray Kelly held up a missing poster of Avonte at the Columbus Day Parade on Monday and said, "We have redoubled our efforts. We have hundreds of police officers and detectives engaged in this search. We've enlisted the help of the guardian angels society, many volunteers."

David Perecman, the Oquendo family's attorney, told CNN last week that he is looking into how Avonte was able to leave school grounds unsupervised.

"Right now, we have submitted a Freedom of Information Act request to the city and the various agencies involved, in order to obtain the information for us to know what occurred," he said.

Avonte's mother says the situation is "heartbreaking."

"I just need to find my son because he needs his family; he cannot fend for himself out there," Vanessa Fontaine told CNN affiliate WABC last week.

"This is just the hardest thing, to have your child disappear and you cannot bring him home with you," she said.

The surveillance video provided by the police department shows that no supervisor or monitor stopped the 14-year-old when he ran out.

"He is supposed to have one-to-one supervision at all times," Fontaine said through tears. "He has the mental capacity of a 7- or 8-year-old."

The New York City Department of Education issued a statement saying it is working closely with police. The school is not commenting.

The Oquendo family filed a "notice of claim" Wednesday, said Perecman, marking the first step of a lawsuit against the city of New York. He declined to give further information about the claim.

Police say Avonte was last seen wearing a gray-striped shirt, black jeans and black sneakers. He is 5 feet, 3 inches tall and weighs 125 pounds.

Missing posters are being handed out, and the surrounding areas are under investigation, WABC reported.

"He doesn't know that, you know, 'I can get hurt in the street, someone can grab me and take me.' He doesn't know that," Fontaine said Monday. "He doesn't know fear."

Anyone with information about Avonte is asked to contact the NYPD's Crime Stoppers Hotline at 800-577-TIPS. The public can also submit tips at the Crime Stoppers website, nypdcrimestoppers.com or text to 274637(CRIMES), then enter TIP577.